The fabrication of small patterned features with a desired functionality is important in a wide variety of fields and applications. Improvements in lithographic techniques to decrease image dimensions have allowed the microelectronics industry to fabricate denser and faster microelectronics chips. Traditional resist technology, however, is rapidly reaching the limits of achievable dimension reductions. Consistent production of sub-60 nm linewidths will require advances beyond the current approaches.
As the size of microelectronic devices shrink, it becomes necessary to define ever smaller features—current manufacturing lines produce sub 70 nm features. Resolving features in this size range requires the use of 193 or 157 nm optical exposures, or electron beam (e-beam) radiation and very thin resist films. Traditional resist films contained phenolic resins that had a moderate resistance to “dry” transfer methods such as reactive ion etching (RIE). Due to the high absorbance of phenolic moieties at wavelengths below 200 nm, resists for deep UV exposure must be primarily aliphatic or cycloaliphatic. The aliphatic polymers used as resist films in the deep UV regime are less etch resistant than traditional phenolic resins. To achieve high resolution, the aspect ratio of the resist thickness to the width of the features must be kept comparable, so it may not be possible to increase the thickness of the resist to compensate for its poorer etch performance.
In addition, the use of high numerical aperture lenses to increase the resolution of optical exposures is reducing the depth of focus of the tools, which also pushes towards the use of very thin resist films. Another problem that occurs when creating fine lithographic features is collapse of resist lines. Even for relatively thin resists (0.1-0.3 um) the wet development step often causes line collapse due to surface tension effects of the aqueous developers and rinses on features with high aspect ratios (Jung, M-H, et al., Proceedings SPIE, Vol. 5039, 1298, 2003).
Line edge roughness is another significant concern in the sub-100 nm feature regime. Traditional resists are composed of polymer chains with “protected” side groups that can react with acids. Photoacid generators produce acids when exposed to light, which then diffuse in the film and catalyze the deprotection of the polymer chain, allowing it to become soluble in the basic developer. The roughness of lithographic features defined by this process can be related to both the size of the polymer chains dissolving out of the film and by the diffusion length of the photoacids (Yoshimura, T., et al., Japan. J. Appl. Phys., Vol. 32, 6065, 1993). For features in the sub-100 nm regime, both of these sources of line edge roughness are important.
Self-assembled monolayers (SAMs). It is well known that organic molecules containing certain terminal head groups will self assemble from solution to form monolayers on specific surfaces (Ulman, A., An Introduction to Ultrathin Organic Films, Academic Press, Chap. 3, 1991). The most common monolayers are formed from organic thiols which attach to gold substrates, organic alkoxy or chloro silanes which react with silicon dioxide, or phosphonic acids, hydroxamic acids, or carboxylic acids which react with metal oxides (Taylor, C. et al., Langmuir, Vol. 19, 2665, 2003). The monolayers are stabilized by the chemisorption of the head group to the surface and the formation of covalent bonds (in the case of silanes or thiols) or ionic bonding (in the case of acids) of the terminal head group with the surface, as well as intermolecular interactions between the molecules such as van der Waals forces, pi-pi interactions or hydrogen bonding.
Self-assembled monolayers are prepared by placing substrates in a solution containing from 0.1 mM to about 1% of the molecules forming the monolayer in a non-reactive, low boiling solvent. The self-assembly process may take from a few minutes up to a day or more to form complete, dense monolayers (Ulman, A., An Introduction to Ultrathin Organic Films, Academic Press, Chap. 3, 1991).
There are various examples of monolayers with terminal tail groups that can bind to metal ions or metal complexes, including phosphonic acids which bind to Zr or Hf (Fang, M., et al., J. Am. Chem. Soc., Vol. 119, 12184, 1997), pyridine which binds to metals or metal complexes such as Rh complexes (Lin, C. et al., J. Am. Chem. Soc., Vol. 125, 336, 2003) or Zr complexes (Hatzor, A. et al., Langmuir, Vol. 16, 4420, 2000), or terpyridine which is capable of binding to a variety of metal ions (Hofmeier, H., et al., Chem. Soc. Rev. Vol. 33, 373, 2004; Maskus, M., et al., Langmuir, Vol. 12, 4455, 1996) The metal/monolayer complexes will self assemble in solution through the chelation of the metal ions/complexes by the tail group of the monolayer.
The initial metal/monolayer complexes may in some cases be extended into multilayer structures through the use of difunctional “linking ligands”, such as diphosphonic acids, dipyridines, diisocyanides or diterpyridines. By sequential exposure to the linking ligand and the metal species, layers may be built up on the original monolayer/metal complex. Films with at least 30 ligand/metal bilayers have been assembled in this fashion (Lin, C. et al., J. Am. Chem. Soc., Vol. 125, 336, 2003).
The concept of using monolayers as ultrathin resists had been proposed and explored by others. Long chain alkyl thiols or silanes have been patterned using UV light or e-beam radiation (Smith, R., et. al., Prog. Surf. Sci., Vol. 75, 1, 2004; Ryan, D., et. al., Langmuir, Vol. 20, 9080, 2004; Zharnikov, M., et. al., J. Vac. Sci. Technol. B, Vol. 20, 1793, 2002; Calvert, J. Vac. Sci. Technol. B, Vol. 11, 2155, 1993). However, the monolayer films that have been proposed to date do not have sufficient RIE etch resistance to transfer images using standard dry etching techniques.